1. Field of the Invention
The present invention relates to a method for detecting failures in wire bonding in wire bonding machines.
2. Prior Art
One example of prior art method for detecting failures in wire bonding is disclosed in Japanese Patent Application Kokai (Laid-Open) No. 59-88842. FIGS. 5 through 7 show this prior method.
As seen in FIG. 5, a lead frame 2 fixed with a pellet 1 is placed at a specific location on a testing table 3.
The bonding wire ("wire") 4 which is to be connected to the pellet 1 and the lead frame 2 is wound on a spool 5. One end of the wire 4 is inserted in the capillary 7 through a clamper 6, and the other end of the wire 4 is connected to a terminal 8. An electric torch 9, that is used for melting the end of the wire 4 to form a ball 4a, is positioned near the capillary 7.
The terminal 8 is connected to a common contact 10a of a first switch 10 which, for example, is a lead relay. A closed contact 10b of the first switch 10 is used for grounding, and an open contact 10c of the first switch 10 is connected to a detector 11 and to resistor 12. The resistor 12 is selectively connected to one of direct current sources 14 and 15 by a second switch 13.
FIG. 6 is a graph showing the upward and downward movements of the capillary 7 and timing of turning of the switch 10. The first switch 10 is turned at the states shown in FIGS. 7(c) and 7(d).
If the pad of the pellet 1 is in the forward direction (see FIG. 2 for "forward direction"), the second switch 13 is connected to the direct-current power supply 14 so as to receive positive voltage. If the pad is in the backward direction (see FIG. 2 for "backward direction"), the second switch 13 is connected to the direct current power supply 15 so as to receive negative voltage.
In operation, first, as seen in FIG. 7(a), the ball 4a is formed at the end of the wire 4, which is placed at the bottom of the capillary 7, by the discharge from the electric torch 9. The capillary 7 is then moved down so that the wire 4 is bonded to the pellet 1 (first bonding) as seen in FIG. 7(b). Then, as seen in FIGS. 7(c), 7(d) and 7(e), the capillary 7 is raised, moved above the second bonding point and then lowered. During this movement, as seen from FIG. 6, the switching signal for the first switch 10 is activated, and the first switch 10 is turned to the open contact 10c.
In this process, if the wire 4 is connected (or bonded) to the pellet 1, a minimal current flows to the wire 4 from the direct-current power supply 14 or 15 via the second switch 13, the resistor 12, the first switch 10 and the terminal 8, but the current does not flow to the detector 11. However, if the wire 4 is not connected (or bonded) with the pellet 1, the current flows to the detector 11 and not to the wire 4. As a result, the wire bonding failure signal 11a is output.
If the wire connection to the pellet is normal (or the bonding has been completed properly at the first bonding point), the capillary 7 is lowered, and the wire 4 is connected to the second bonding point as seen in FIG. 7(e). Then, as seen in FIG. 7(f), the clamper 6 is closed, and only the capillary 7 moves up to a predetermined height. After this, as seen in FIG. 7(g), both the clamper 6 and the capillary 7 are moved up together, and the wire 4 is cut at the second bonding point.
The wire 4b is connected to the first bonding point and the second bonding point as described above. Then the capillary 7 is moved upward until it reaches above the first bonding point at which next bonding is performed, and as seen in FIG. 7(a), the ball 4a is formed, and the process described above will be repeated.
However, the problem in the prior art method described above is that only the failure in the first bonding (or non-bonding) is detected and not in the second. Thus, bonding reliability is inferior.